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Re: sauropods: homotherm,heterotherm or gigantotherm?



--- David Marjanovic <david.marjanovic@gmx.at> wrote:

> > Abstract: "[...] crocodilians retain a
> four-chambered heart.  However, 
> > crocodilians have a neurally controlled, pulmonary
> bypass shunt that is 
> > functional in diving.  Shunting occurs outside of
> the heart and involves 
> > the left aortic arch that originates from the
> right ventricle, the foramen 
> > of Panizza between the left and right aortic
> arches, and the cog-tooth 
> > valve at the base of the pulmonary artery. 
> Developmental studies show 
> > that all of these uniquely crocodilian features
> are secondarily derived, 
> > indicating a shift from the complete separation of
> blood flow of 
> > endotherms to the controlled shunting of
> ectotherms.  We present other 
> > evidence for endothermy in stem archosaurs and
> suggest that some dinosaurs 
> > may have inherited the trait."
> 
> I still haven't read that (because I haven't
> stumbled across the journal --  
> the library here should have it, however), but...
> that developmentary 
> evidence must be interesting. We have a foramen of
> Panizza before it closes 
> at birth (allegedly due to the sudden pressure of
> the first cry).

++++++++++++++++++++++++++++++++++

The foramen ovale is not equivalent to the foramen of
Panizza. The foramen of Panizza is located OUTSIDE of
the actual heart. It occurs between the two aortae.
The foramen ovale in humans, occurs INSIDE the heart,
between the two atria.

As for the Bennet-Stamper et al article, all I really
gathered from it was this basic argument:

.) Early crocodylomorphs were more active than extant
crocodylomorphs

.) Activity requires endothermy.

.) Therefore early crocodylomorphs were likely to be
endothermic.

As the second assumption has never been experimentally
born out (with plenty extant examples to the
contrary), I don't really buy it. 

Plus I felt they just glossed over the heart
morphology of pythons and varanids in an attempt to
explain why crocodylians have 4 chambered hearts (both
varanid & python hearts [possibly more species as
well] are functionally 4-chambered, and capable of
pressure separations equivalent to those seen in
mammals & birds).

___________________________


> >>There is one known living gigantotherm: the
> leatherback turtle 
> >>(*Dermochelys coriacea*).
> >
> > Tuna and some sharks (e.g., great white shark,
> makos) also have
> > some kind of endothermy ("regional endothermy"),
> 
> Yes, but that's not gigantothermy. Those tunas (not
> all) and sharks do have 
> increased metabolic rates. 

+++++++++++++++++++++++++++++++++

No they don't. They use the same counter-current heat
exchange system that _D.coriacea_ uses. They're just
not as "fatty" (makes sense since they don't usually
enter water as cold as that of _D.coriacea_'s). 

Besides that, there is at least one study out there
(that isn't Paladino et al's infamous one), which
shows that the metabolism of _D.coriacea_ does
increase when in colder waters.

Davenport, J., 1998. Sustaining endothermy on a diet
of cold jelly: energetics of the Leatherback Turtle
Dermochelys coriacea. Brit. Herpetol. Soc. Bull. 62:
4--8.
 
If one is going to insist that _Dermochelys_ is a
gigantotherm, then tuna and lamniforme sharks might as
well be lumped in there too.

Jason

"I am impressed by the fact that we know less about many modern [reptile] types 
than we do of many fossil groups." - Alfred S. Romer


        
                
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